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Taxon, taxa: a taxonomic unit (in principle any taxonomic unit), most often used for either a subspecies or a species. For example, when discussing Kaempfer's Woodpecker calling it "the taxon obrieni" does not make a statement as to whether Kaempfer's Woodpecker is a full species or a subspecies.

Taxonomic order: a linear order of the species of birds of the world based on a phylogenetic analysis (see Phylogeny), and therefore also based on the evolutionary relationship among the birds. Bird families that are very old are placed first, more recent additions later. The commonly used order has for a long time been based on Voous 19771 but some recent DNA based studies have revolutionised the order: one example of the results are that ducks and gamebirds have been moved to near the start of the sequence just after ratites, and grebes and flamingos are each others closest relatives.

Taxonomy: the study of how different bird populations relate to each other in an evolutionary sense.

Tomia: The cutting edge of a birds beak, in some wildfowl this is serrated like the teeth of a saw. In some raptors there is a notch on the upper mandible just behind the hook of the beak, this is known as the tomial tooth, also known as the 'notch'. It is used to break the neck of their prey.

Transequatorial: something that goes across the equator. When used about birds, usually it is referring to transequatorial migration, where a species spends its breeding period on one side and migrate to the other side of the equator. One example is the Arctic Tern that moves from breeding in northern summer to far southern areas where it spends its non-breeding season.

Trapline, traplining: used to describe a route which takes a hummingbird to the same flowers (or flower groups) in the same order; each flower may be visited one to several times daily. This strategy are used more often in hermits of genus Phaethornis but also in some other species of hummingbirds.

Type location: the location where a particular type specimen was collected.

Type species: in defining a genus, the type species is the species selected to define the genus by. If a genus is split into two genera, the new genus which includes the type species of the old genus, retains the same name. The selection of a type species is done by ornithologists on a "first come, first served" basis (called priority) - if two ornithologists select different type species for a genus, the one who published first has their type species accepted.

Type specimen: the individual specimen of a bird, stored in a museum, which defines its species or subspecies.

Undulating: usually used about flight where the bird alternates upwards active flight with downwards gliding, so that the overall flight pattern reminds of a sinus curve. Especially common in e.g., Woodpeckers and several groups of smaller birds where the downwards glide may involve closed wings.

Uropygial glands: a pair of glands situated at the base of the tail (upperside) in many but not all birds. These glands produce a secretion that is distributed over the feathers of the bird in the preening process, and the glands are therefore also called "preening glands". The secretion from these glands have antibacterial and antifungal activities, and it may contain a precursor for vitamin D that is converted by sunlight. In many species of birds, the secretion is oily and confer waterproofing to the feathers (for example in ducks). In some species the glands open to the outside so close to each other that they can seem like a single gland, which is why one will see them mentioned as the "Uropygial gland". See this excellent photo in the birdforum gallery

Vermiculated: finely marked with wavy patterns, often only visible at close range. Most often used about patterns on birds (in feathers) but may also apply to eggs.

Vicariance: the situation when two closely related populations get separated, for example by ice during an ice age,
by water such as a river or sea, a mountain range, etc. Some theories of speciation state that vicariance is a necessary step in producing two species from populations that were one species before.

Wattle, Wattles: a name given to structures found on some birds, usually on the throat, chin, or head. These structures consist of skin and possibly other structures below the skin, and they usually are not covered in feathers. The colour of the wattles often contrasts with the structure below, often as bright colours such as red. Wattles often seems to have function in display and reproduction behaviour, as wattles may be reduced in size or brightness outside of breeding season. See for example Wattled Guan or Bearded Bellbird.

Whiffling: the name given to the method of rapid descent that some waterfowl use. They twist their body and wings, first one way then the other, loosing lift and forward speed in the process to very quickly descend to the water surface. When large flocks of birds engage in this manouver the result can be astounding.

World-wide checklists: for purposes of taxonomy in the Opus, the baseline is the latest Clements checklist1. Opus will change if there is a consensus between the Howard & Moore checklists2 and the latest list of the IOC3. One additional list is worth mentioning: the BirdLife international list published together with the Handbook of Birds of the World.

Zygodactyl: Having two toes on each foot pointing forwards whilst the other two point backwards. The backwards pointing toes are the innermost and outermost toes. Woodpeckers and other arboreal species share this trait while Trogons have a Heterodactyl arrangement. see Legs and Feet

Zugstimmung: behavioral changes coming before and preparing the bird for its migration: for example leading to build-up of fat reserves that allow for long, unbroken migratory flights and also changing day/night pattern of activity (the actual change in body composition is known as Zugdisposition). The word is German but adopted into English language technical literature.

Zugunruhe: restlessness and activity displayed by a captive bird of a migratory species during the period it should have been actively migrating. Some scientific studies have used the Zugunruhe to successfully determine migratory direction in birds by capturing migrants, hold them in captivity and determine the direction they are trying to move in a specifically designed funnel. Zugunruhe is German but adopted into English language technical literature; also called migratory restlessness. (Sometimes this term is used more broadly, including the behavior here listed as Zugstimmung (see above)).